Automatic adjustment of receiver apparatus based on channel-bit-error-rate-affected parameter measurement

ABSTRACT

Alignment of an antenna and a polarizer for the antenna are automatically adjusted and the frequency setting of a frequency-tunable receiver is automatically adjusted while receiving a broadcast communication signal that is forward-error-correction coded by processing the forward-error-correction code of the received signal to measure a channel-bit-error-rate-affected parameter for the received signal; by realigning the antenna and the polarizer from their respective initial positions in response to the measured parameter until the measured parameter is minimized; and by retuning the receiver from its initial frequency setting in response to the measured parameter unitl the measured parameter is minimized. A Viterbi decoder processes the forward-error-correction code of the received signal to compute path metrics affected the bit-error rate of the received signal and normalizes the computed path metrics; and a counter counts the normalizations over fixed intervals to measure a path metric normalization rate as the measured parameter.

BACKGROUND OF THE INVENTION

The present invention generally pertains to broadcast signal receivingsystems and is particularly directed to a system for automaticallyadjusting an apparatus for receiving a broadcast communication signalwhile the broadcast signal is being received. Such receiving apparatusinclude apparatus that may be adjusted to effect reception of abroadcast communication signal, such as an antenna, an antenna polarizerand a frequency-tunable receiver.

Typically, an antenna is attached to an antenna mount by an actuator andis rotated about a given axis on the antenna mount by moving theactuator in order to align the antenna with a given signal source, suchas a communication satellite in geosynchronous orbit above the Earth'sequator.

Once the antenna is aligned with the given signal source, it may also benecessary to align the polarizer of the antenna so that a linearpolarization axis of the antenna matches a linear polarization axis of acommunication channel being received from the signal source. Alignmentof the polarizer is required when the signal source is a communicationsatellite, since odd and even numbered channels received fromcommunication satellites are skewed ninety degrees with respect to eachother in order to avoid interference between adjacent channels. One typeof polarizer is aligned by rotating a feed probe within the polarizer.

The antenna and the polarizer are respectively realigned from theirpresent positions in response to control signals provided by an antennacontroller.

A typical frequency-tunable receiver is tuned to provide the receivedsignal at a given intermediate frequency setting by adjusting thefrequency of a local oscillator signal in response to operation of acontrol device to select a broadcast channel so that when the localoscillator signal is mixed with a received broadcast communicationsignal that is at the carrier frequency of the selected channel, thereceived signal is provided at the given intermediate frequency.

When one desires to change the received communication channel, thereceiver is tuned to the carrier frequency of a newly selected broadcastchannel, and an identification of the newly selected channel is providedto the antenna controller, which automatically effects a coarserealignment of the antenna and/or the polarizer, when the new channel isreceived from a different signal source and/or has a different linearpolarization axis than the last previously received broadcast signal.Such realignment is effected in accordance with information stored in adata memory of the controller indicating certain alignment parameters ofeach of a plurality of channels.

In some prior art antenna controllers, fine alignment of the antenna andthe polarizer are automatically achieved once the communication signalis being received following coarse realignment. Such fine alignment isachieved by measuring the amplitude of the received signal andrealigning the antenna and/or the polarizer from an initial positionuntil the measured signal amplitude is maximized. However, wheninterference and/or noise also are received over the communicationchannel, optimum signal reception is not always achieved at the maximummeasured signal amplitude.

In some receivers, fine tuning to a given intermediate frequency isautomatically achieved by a feedback technique, such as afrequency-locked loop. However, when interference and/or noise also arereceived over the communication channel, optimum signal reception is notalways achieved at precisely such an automatically fine-tuned frequency.

These are particular problems with video signal reception, sincedifferences in the quality of video reception are readily perceived byviewers.

SUMMARY OF THE INVENTION

The present invention provides an improved system for automaticallyadjusting a receiving apparatus to optimize reception of a broadcastcommunication signal that is forward-error-correction coded, comprisingmeans for processing a received forward-error-correction-code of thereceived signal to measure a channel-bit-error-rate-affected parameterfor the received signal; and means for adjusting the apparatus from aninitial setting in response to said measurement until the measuredparameter is minimized.

By adjusting such receiving apparatus as an antenna, an antennapolarizer and/or a frequency-tunable receiver to minimize the measuredparameter the quality of signal reception is opimized in the presence ofinterference and noise.

Additional features of the present invention are described in relationto the description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of the system of the present invention incombination with an antenna.

FIG. 2 is a diagram of a combination of processing elements implementedby the processor in the system of FIG. 1.

FIG. 3 is a diagram of a processing routine executed by the processor inthe system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, in a preferred embodiment of the present invention,a controller 10 is coupled to a frequency-tunable communication signalreceiver 11, an actuator 12 for an antenna 14 and to a mechanicalpolarizer 16 for the antenna 14. The controller 10 includes a memory 18,a keypad 20 and a processor 22. The controller 10 and the communicationsignal receiver 11 are housed in a common chassis 26, except that thecontroller keypad 20 is contained in a remote control unit. The memory18 stores antenna alignment parameters, polarizer alignment parametersand frequency-setting data for each of a plurality of predeterminedcommunication channels. The receiver 11 is tuned by adjusting thefrequency of a local oscillator signal that is mixed with a receiversignal at the carrier frequency of the selected channel to provide thereceived signal at a given intermediate frequency.

When a different receiver channel is selected by operation of the keypad20, the processor 22 executes a coarse-adjustment processing routine 27that causes the controller 10 to effect a coarse alignment of theantenna 14 and the polarizer 16 in accordance with the alignmentparameters stored in the memory 18 for the selected channel by providingcontrol signals on line 28 to the actuator 12 and on line 30 to thepolarizer 16, and also causes the controller 10 to tune the receiver 11to an initial frequency setting in accordance with frequency-settingdata stored in the memory 18 for the selected channel by providing acontrol signal on line 31 to the receiver 11.

The receiver 11 is tuned to provide the received signal at a givenintermediate frequency setting by adjusting the frequency of the localoscillator signal in response to the control signal on line 31 so thatwhen the local oscillator signal is mixed with a received broadcastcommunication signal that is at the carrier frequency of the selectedchannel, the received signal is provided at the given intermediatefrequency.

Once a signal that has been forward-error-correction coded is receivedby the receiver 11 on line 32 from the polarizer 16, the processor 22executes a measurement-and-readjustment processing routine 34 ofprocessing the received signal to measure the channel-bit-error-rateaffected parameter of the received signal and causes the controller 10to provide control signals on line 28 to the actuator 12, on line 30 tothe polarizer 16 and on line 31 to the receiver 11 to realign theantenna 14 and the polarizer 16 and to retune the receiver 11 until themeasured bit-error-rate-affected parameter is minimized. Preferably, themeasurement-and-readjustment routine 34 is executed sequentially for theantenna 14, the polarizer 16 and the receiver 11, rather thansimultaneously.

In embodiments of the invention designed for use with systems thatreceive communication signals that are forward error corrected with aconvolutional code, the processor 22 in the system of FIG. 1 includes aViterbi decoder 35 and a counter 36, as shown in FIG. 2.

The Viterbi decoder 35 processes the forward-error-correction codesymbols of the received signal to compute path metrics affected by thebit-error rate of the received signal and normalizes the computed pathmetrics. The counter 36 counts the normalizations 37 by the Viterbidecoder 35 over fixed intervals defined by a periodic reset signal 38 toprovide a measurement 39 of the path metric normalization rate as themeasured channel-bit-error-rate-affected parameter for the receivedsignal. The counter 36 responds to each reset signal 38 by providing theaccumulated count of normalizations and resetting the count to zero.Utilization of the path metric normalization rate as the measuredchannel-bit-error-rate-affected, parameter rapidly provides accuratemeasurements over a wide range of C/N values for enabling rapid accurateautomatic alignment of the antenna 14.

The measurement-and-readjustment processing routine 34 includes a seriesof steps 40, 42 and 44, as shown in FIG. 3.

When the processing routine 34 is performed to readjust the alignment ofthe antenna 14 or the polarizer 16, in the first step 40, thechannel-bit-error-rate-affected parameter for the received signal ismeasured at each of a plurality of positions of the antenna 14 and thepolarizer 16 extending to N positions on both sides of the respectiveinitial position P of the antenna 14 or the polarizer 16 when theprocessing routine 34 begins. The measurement range extends from aposition P-N on one side of the position P to position P+N on the otherside of the position P.

In the second step 42 of readjusting the alignment of the antenna 14 orthe polarizer 16, the controller 10 realigns the antenna 14 or thepolarizer 16 until the measured parameter is minimized. The measurementstep 40 and the readjustment step 42 are repeated for each positionwithin the measurement range, with the antenna 14 or the polarizer 16being realigned after each measurement of thechannel-bit-error-rate-affected parameter. Once it is determined atwhich position the measured parameter is minimized, realignment of theantenna 14 or the polarizer 16 ceases when the antenna 14 or thepolarizer 16 is realigned to its minimized-measured-parameter position.

In the third step 44 of readjusting the alignment of the antenna 14 orthe polarizer 16, it is determined whether the position within themeasurement range at which the measured parameter is minimized byrealignment is at a boundary the measurement range, P+N or P-N. If so,the measurement steps 40 and the readjustment steps 42 are repeated fora new measurement range extending N positions on both sides of the aboundary of the previous measurement range at which the minimum value ofthe channel-bit-error-rate-affected parameter was measured.

When the processing routine 34 is performed to readjust the tuning ofthe receiver 11, in the first step 40, thechannel-bit-error-rate-affected parameter for the received signal ismeasured at each of a plurality of frequency settings of the receiver11, extending to N frequency settings on both sides of the initialfrequency setting when the processing routine 34 begins. The measurementrange extends from a setting P-N on one side of the setting P to asetting P+N on the other side of the setting P.

In the second step 42 of readjusting the tuning of the receiver 11, thecontroller 10 retunes the receiver 11 in response to the control signalon line 31 by adjusting the frequency of the local oscillator signal tothereby change the intermediate frequency of the receiver 11 until themeasured parameter is minimized. The measurement step 40 and thereadjustment step 42 are repeated for each setting within themeasurement range, with the receiver 11 being retuned after eachmeasurement of the channel-bit-error-rate-affected parameter. Once it isdetermined at which frequency setting the measured parameter isminimized, retuning of the receiver 11 ceases when the receiver 11 isreturned to its minimized-measured-parameter frequency setting.

In the third step 44 of readjusting the tuning of the receiver 11, it isdetermined whether the frequency setting within the measurement range atwhich the measured parameter is minimized by retuning is at a boundaryof the measurement range, P+N or P-N. If so, the measurement steps 40and the readjustment steps 42 are repeated for a new measurement rangeextending N settings on both sides of the boundary of the previousmeasurement range at which the minimum value of thechannel-bit-error-rate-affected parameter was measured.

Although the adjustment system of the present invention is particularlyuseful when receiving digital video signals with a satellitecommunication system, it also is useful with other types of digitalcommunications systems for receiving other types of digitalcommunication signals.

We claim:
 1. A system for atuomatically adjusting an apparatus forreceiving a broadcast communication signal that isforward-error-correction coded, comprisingmeans for processing areceived forward-error-correction-coded signal to measure achannel-bit-error-rate-affected parameter for the received signal; andmeans for adjusting the apparatus from an initial setting in response tosaid measurement until the measured parameter is minimized.
 2. A systemaccording to claim 1, wherein the adjusting means are adapted forreadjusting the apparatus within a given range of settings defined byextreme settings on both sides of the initial setting, for extendingsaid given range beyond one of said boundaries when the measuredparameter is minimized at said one boundaries and for readjusting saidapparatus within said extended given range.
 3. A system according toclaim 1, wherein the processing means includesa Viterbi decoder forprocessing the forward-error-correction code of the received signal whenthe received signal has been forward error corrected with aconvolutional code, to compute path metrics affected by the bit-errorrate of the received signal and for normalizing said computed pathmetrics, and means for counting said normalizations over fixed intervalsto measure a path metric normalization rate as the measured parameter.4. A system according to claim 1, wherein the apparatus is an antenna;andwherein the adjusting means are responsive to said measurement forrealigning the antenna from an initial position until the measuredparameter is minimized.
 5. A system according to claim 4, wherein theadjusting means are adapted for realigning the antenna within a givenrange of positions defined by extreme positions on both sides of theinitial position, for extending said given range beyond one of saidboundaries when the measured parameter is minimized at said oneboundaries and for realigning said antenna within said extended givenrange.
 6. A system according to claim 1, wherein the apparatus is anantenna polarizer; andwherein the adjusting means are responsive to saidmeasurement for realigning the polarizer from an initial position untilthe measured parameter is minimized.
 7. A system according to claim 6,wherein the adjusting means are adapted for realigning the polarizerwithin a given range of positions defined by extreme positions on bothsides of the initial position, for extending said given range beyond oneof said boundaries when the measured parameter is minimized at said oneboundary and for realigning said polarizer within said extended givenrange.
 8. A system according to claim 1, wherein the apparatus is afrequency-tunable receiver; andwherein the adjusting means areresponsive to said measurement for retuning the receiver from an initialfrequency setting until the measured parameter is minimized.
 9. A systemaccording to claim 8, wherein the adjusting means are adapted forreturning the receiver within a given range of frequency setting definedby extreme frequency settings on both sides of the initial setting, forextending said given range beyond one of said boundaries when themeasured parameter is minimized at said one boundary and for retuningsaid receiver within said extended given range.